Fuel cells are classified into polymer electrolyte (solid polymer) fuel cells, phosphoric acid fuel cells, alkaline fuel cells, molten carbonate fuel cells, solid oxide fuel cells, etc. according to the kind of the electrolyte used. Among them, polymer electrolyte fuel cells (PEFCs) are becoming commercially available as the power source for automobiles, home cogeneration systems, etc, because they operate at low temperatures and have high output densities.
Recently, the use of fuel cells as the power source for portable small electronic devices, such as notebook personal computers, cellular phones, and personal digital assistants (PDAs), has been examined. Fuel cells can generate power continuously if they get refueled. Thus, the use of fuel cells in place of secondary batteries which need recharging is expected to improve the convenience of portable small electronic devices. Also, PEFCs are advantageous as the power source for portable small electronic devices due to the low operating temperature as mentioned above. Fuel cells are also becoming commercially available as the power source in outdoor leisure activities such as camping.
Among PEFCs, direct oxidation fuel cells (DOFCs) use a fuel that is liquid at room temperature, and generate electrical energy by directly oxidizing the fuel without reforming it into hydrogen. Thus, direct oxidation fuel cells do not require a reformer and can be easily miniaturized.
Among direct oxidation fuel cells, direct methanol fuel cells (DMFCs), which use methanol as the fuel, are superior in energy efficiency and output power to other direct oxidation fuel cells. They are thus regarded as the most promising power source for portable small electronic devices.
The reactions of DMFCs at the anode and the cathode are represented by the following reaction formulae (11) and (12), respectively. Oxygen introduced into the cathode is usually sucked from the air.Anode: CH3OH+H2O→CO2+6H++6e  (11)Cathode: (3/2)O2+6H++6e−→3H2O  (12)
The technical problems of polymer electrolyte fuel cells are described below.
In a catalyst layer of an electrode included in a polymer electrolyte fuel cell, a three-phase interface between a phase in which a reactant is transported, a phase in which ions are conducted, and a phase in which electrons are conducted is an active site. It is very important to form a three-phase interface efficiently in order to increase output and power generation efficiency or reduce the amount of catalyst to reduce costs.
In order to form a three-phase interface efficiently, Japanese Laid-Open Patent Publication No. 2001-85033 (hereinafter “Patent Document 1”) proposes forming a plurality of ridges and a plurality of grooves between the ridges on a surface of an electrode substrate or a solid polymer electrolyte membrane, and attaching a catalyst to the ridgelines where the top faces of the ridges and the walls of the grooves intersect.
Also, in order to supply a fuel or an oxidant to the three-phase interface efficiently, Japanese Laid-Open Patent Publication No. 2008-41488 (hereinafter “Patent Document 2”) proposes providing a diffusion layer with through-holes penetrating the thickness thereof, and providing a catalyst layer with grooves that form flow channels in the plane direction thereof. It proposes disposing a catalyst in the positions corresponding to the through-holes of the diffusion layer so that the fuel or oxidant having passed through the through-holes can be smoothly supplied to the catalyst layer.
The techniques disclosed in Patent Documents 1 and 2 are effective for forming a three-phase interface efficiently and supplying the fuel or oxidant smoothly. However, according to these techniques, the catalyst layer is formed only on part of the electrolyte membrane. Thus, the technical problems of polymer electrolyte fuel cells described below may be aggravated, and an overall improvement in output and power generation efficiency may not be achieved.
The first problem relates to an improvement in output density per unit (projected) area. Generally, a fuel cell is used in the form of a fuel cell stack comprising a plurality of cells stacked in series. Thus, the output density of the fuel cell stack increases with increasing output per unit (projected) area of electrode in the stacking direction of the cells, i.e., the area-based output density. An increase in output density allows reductions in the size, weight, or costs of the fuel cell system.
To increase the output density per unit area, it is also important to increase the number of three-phase interfaces and supply a fuel and an oxidant smoothly. However, the active site in a catalyst layer to which protons are supplied most smoothly is the interface between the electrolyte membrane and the catalyst layer closest to the electrolyte membrane. Patent Documents 1 and 2 disclose a cell structure in which part of the interface has no catalyst layer. With such a cell structure, the electrolyte membrane cannot be effectively utilized, and it is difficult to increase the output density.
The second problem is a problem characteristic of direct oxidation fuel cells such as DMFCs. That is, there is a need to prevent a liquid fuel (e.g., an aqueous methanol solution) supplied from the fuel flow channel from permeating the anode and the electrolyte membrane, reaching the cathode, and being oxidized in the cathode catalyst layer. This phenomenon is called fuel crossover, and in the case of DMFCs, it is called methanol crossover (MCO). Such a phenomenon occurs because a water-soluble liquid fuel is often used. A water-soluble liquid fuel tends to permeate the electrolyte membrane that tends to absorb water.
Such fuel crossover lowers the fuel utilization efficiency because the fuel is not consumed at the anode. Further, the oxidation reaction of the crossover fuel at the cathode conflicts with the cathode reaction, i.e., the reduction reaction of the oxidant (oxygen) at the cathode, thereby lowering the cathode potential. This results in a decrease in cell voltage and power generation efficiency.
To solve this problem, for example, in DMFCs, electrolyte membranes that allow little methanol to permeate therethrough are being actively developed to reduce MCO. However, currently available electrolyte membranes conduct protons through water present in the membranes, and thus, the electrolyte membranes require water. Also, methanol has high affinity for water. It is thus difficult to sufficiently prevent methanol from permeating the electrolyte membrane together with water
The movement of liquid fuel inside the electrolyte membrane is due mainly to concentration diffusion. It is thus known that the degree of fuel crossover is significantly dependent on the difference in fuel concentration between the anode-side surface and the cathode-side surface of the electrolyte membrane. The fuel concentration on the cathode-side surface of the electrolyte membrane is believed to be negligibly small, because the crossover fuel is promptly oxidized at the cathode. Thus, after all, the amount of fuel crossover is significantly dependent on the fuel concentration on the anode-side surface of the electrolyte membrane.
In the structures disclosed in Patent Documents 1 and 2, part of the electrolyte membrane is not in contact with the anode catalyst layer or the anode diffusion layer. In such a structure, the fuel supplied to the fuel flow channel directly reaches the surface of the electrolyte membrane without being consumed in the anode catalyst layer. As a result, the fuel concentration on the anode-side surface of the electrolyte membrane increases and the amount of fuel crossover increases.